The Supreme Court gave its judgment in R (on the application of Miller and another) v the Secretary of State for Exiting the European Union on 24 January 2017.

This was the government's appeal against the judgment of the Divisional Court, given in November, that the Secretary of State did not have power under the royal prerogative to give notice that the United Kingdom would leave the European Union.

For the first time in the Supreme Court's history, all eleven justices sat to hear the appeal. By a majority of eight to three, they upheld the Divisional Court's decision. This article summarises the Supreme Court's judgment.

The majority judgment

The leading judgment was given by Lord Neuberger (the President of the Supreme Court), Lady Hale, Lord Mance, Lord Kerr, Lord Clarke, Lord Wilson, Lord Sumption and Lord Hodge.

Their Lordships summarised the question for the court as whether ministers could give formal notice of withdrawal from the EU without prior legislation being passed by Parliament.

This issue arises from two features of the British constitution. On the one hand, ministers may use the royal prerogative to enter into and terminate international treaties without recourse to Parliament. On the other hand, ministers may not normally exercise any power resulting in a change in domestic law unless an Act of Parliament so provides.

The European Communities Act 1972 and subsequent Acts

British membership of the European Economic Community, which became the European Union, was given effect by the European Communities Act 1972. Section 2 of that Act provides, in summary, that all such rights arising under the European treaties as are without further enactment to be given legal effect in the UK are to be recognised and available in law. Thus the 1972 Act makes EU law effective in the UK, and so changes domestic law.

The 1972 Act has since been amended so as to refer to subsequent European treaties. In particular, the European Union (Amendment) Act 2008 added the Treaty of Lisbon, which includes, as article 50 of the Treaty on European Union, a provision by which a member state may decide to withdraw from the EU in accordance with its own constitutional requirements. The European Union Act 2011 provides that any treaty amending or replacing the European treaties, and any decisions resulting in a dilution of the influence of the member states, may not be ratified without the prior approval of Parliament.

The government's case

The government's case was that the 1972 Act does not exclude the use by ministers of prerogative power to withdraw from the EU treaties, since it gives effect to EU law only insofar as the European treaties require it. The effect of EU law in the UK is contingent on the UK remaining party to the treaties. While subsequent statutes have required parliamentary approval for any enlargement of EU powers, they have not affected the prerogative power to withdraw from the EU altogether. Because the result of the EU referendum last June was a vote to leave, that power should now be used.

The constitutional background and the royal prerogative

The royal prerogative is a power exercised by ministers on behalf of the Crown. Over the years, many prerogative powers have been reduced by statute, but powers relating to foreign treaties and the conduct of diplomacy and war continue to be reserved to ministers. It is, however, a settled constitutional principle that any exercise of prerogative powers by ministers must be compatible with the law: ministers may not use the prerogative to change domestic law or to prevent a statute from operating as Parliament intended.

The use of the prerogative in foreign affairs, in particular the making and unmaking of treaties, is consistent with the rule that ministers cannot alter domestic law because international law and domestic law are regarded as operating in independent spheres: treaties between states have effect in international law and are not governed by domestic law, nor do they give rise to rights or obligations in domestic law. If an international treaty is to have any consequences in domestic law, it is for Parliament to legislate to bring the necessary changes into effect.

The status and character of the 1972 Act

The effect of the European Communities Act 1972 is to constitute EU law as an automatic and overriding source of law in the UK. For as long as the UK is a member of the EU, the EU treaties and regulations are directly applicable and EU directives are implemented by way of delegated legislation. Further, any domestic legislation must be consistent with EU law or it will be ineffective.

This does not, however, prevent Parliament from deciding, if it repeals the 1972 Act, that EU law should no longer have effect in the UK. If the UK withdraws from the EU, the legal effect will be that, while the directives transposed into UK law will remain in force, the EU treaties and regulations, and the rights they confer, will no longer be applicable.

Conclusion

Their Lordships held that the 1972 Act does not accommodate or authorise the abrogation of EU law on the UK's withdrawal from the EU without the prior authorisation of Parliament. On the contrary, the 1972 Act gave effect to UK membership of the EU in a way that is inconsistent with the use of any prerogative power to withdraw from the EU treaties.

While the rights and remedies incorporated into domestic law from EU law vary as EU law changes and new EU legislation comes into effect, withdrawal from the EU is fundamentally different from variations in the content of EU law - as significant a constitutional change as when EU law was incorporated in domestic law by the 1972 Act.

Such a far-reaching change to the UK constitutional arrangements cannot be brought about by the use of prerogative powers, especially when the source of law was brought into existence by primary legislation in the first place.

Since the statute authorising last year's EU referendum did not provide for any legal consequences of the vote to withdraw, it will be necessary for a further Act of Parliament to be passed before notification to leave the EU can be given.

The dissenting judgments

Lord Reed, with whom Lord Carnwath and Lord Hughes agreed, would have allowed the government's appeal.

In his view, the effect given to EU law in domestic law by the 1972 Act is inherently conditional on the UK's membership of the EU. The Act does not therefore affect the Crown's exercise of prerogative powers in relation to the UK's EU membership.

Lord Reed did not accept the distinction drawn in the majority judgment between changes in domestic law resulting from variations in the content of EU law and changes resulting from withdrawal from the EU. Parliament granted the rights given by EU law on the basis that they would expire on leaving the EU, and so those rights would be revoked by the operation of the 1972 Act itself.

In further dissenting judgments, Lord Carnwath added that the EU statutes did not restrict the executive's power to make or withdraw from an international treaty. Legislation would be needed to implement withdrawal, but not to initiate it. Lord Hughes considered that giving notice under article 50 would not change domestic law, because on withdrawal the 1972 Act would cease to operate by its own terms.

These views did not, however, persuade the majority of the Supreme Court that the Secretary of State's prerogative power could be used in this way.

The devolution question

As to the role of the devolved legislatures, it was held unanimously that relations with the EU, like other matters of foreign affairs, are reserved to the UK government and not devolved to Scotland, Wales and Northern Ireland. The devolved legislatures do not have a legislative competence parallel to that of Westminster in relation to withdrawal from the EU and cannot veto the decision to leave. The Sewel Convention, by which Westminster would not normally legislate with regard to devolved matters except with the agreement of the devolved legislature, was considered to be a matter of politics rather than of law.

Consequences of the judgment

The government has indicated that, because the court has held that the process of leaving the EU cannot be initiated by the use of prerogative power, it will introduce a parliamentary Bill to authorise notification of the UK's intention to withdraw. Thus the giving of notice to leave will be a matter to be decided by Act of Parliament, as the Supreme Court has determined.

R (on the application of Miller and another) v the Secretary of State for Exiting the European Union [2017] UKSC 5